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McKenney" cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, rostedt@goodmis.org, Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Vlastimil Babka , Roman Gushchin , Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org Subject: Re: [PATCH rcu 5/8] slab: Explain why SLAB_DESTROY_BY_RCU reference before locking In-Reply-To: <20221019224659.2499511-5-paulmck@kernel.org> Message-ID: <03d5730-9241-542d-76c6-728be4487c4@gentwo.de> References: <20221019224652.GA2499358@paulmck-ThinkPad-P17-Gen-1> <20221019224659.2499511-5-paulmck@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666338266; a=rsa-sha256; cv=none; b=GFqwWkf9tq+Ao3fJbqam2Nq1kIJ8naE9w4h9o9nwJ12lj966ZlIRs2Pa9ajexfFbVWlkm+ LHhwYBHisHn6hBwlqHcf/f0OYPo4mw4+GrDOT86nwfV02/sT7VYfDfyn98QrIMW2nC6H/V 1RWqP4EB29QF5AbvETxEnNJBa7uFamE= ARC-Authentication-Results: i=1; imf07.hostedemail.com; dkim=none ("invalid DKIM record") header.d=gentwo.de header.s=default header.b=A32PqipN; dmarc=pass (policy=none) header.from=gentwo.de; spf=pass (imf07.hostedemail.com: domain of cl@gentwo.de designates 161.97.139.209 as permitted sender) smtp.mailfrom=cl@gentwo.de ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666338266; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=BvcmoE149ifL+s+NnSFOE5DnO9bCZ83e1JWjimwePZI=; b=ZDj3YUzh5ciXoE53me6J+dP8nggaHwiItrYBLkRKUnivjONfn7Xk+PsCXn2Q49694fa7p5 cRYXv13EEzODD6Yb+x8mLKZdIjiyyuBL9IgYsGZHlLpklp6Dtv6dSbf3uQ0fROoBcsk8Rx 64YY1fZgl3lURLHXChhg2m9g4h0I2TA= Authentication-Results: imf07.hostedemail.com; dkim=none ("invalid DKIM record") header.d=gentwo.de header.s=default header.b=A32PqipN; dmarc=pass (policy=none) header.from=gentwo.de; spf=pass (imf07.hostedemail.com: domain of cl@gentwo.de designates 161.97.139.209 as permitted sender) smtp.mailfrom=cl@gentwo.de X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: q3yojid4nu1r4uy39cn4gpdpneda1oaf X-Rspamd-Queue-Id: EA5344000A X-HE-Tag: 1666338265-798454 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 19 Oct 2022, Paul E. McKenney wrote: > It is not obvious to the casual user why it is absolutely necessary to > acquire a reference to a SLAB_DESTROY_BY_RCU structure before acquiring > a lock in that structure. Therefore, add a comment explaining this point. Sorry but this is not correct and difficult to comprehend. 1. You do not need a reference to a slab object after it was allocated. Objects must be properly protected by rcu_locks. 2. Locks are initialized once on slab allocation via a constructor (*not* on object allocation via kmem_cache_alloc) 3. Modifying locks at allocation/free is not possible since references to these objects may still persist after free and before alloc. 4. The old term SLAB_DESTROY_BY_RCU is used here.